@Ayengar 

For item 5, what happens if you configure the same SSID/password for the two sets of bands?

  @Ayengar 

Don't know if this is any help but I will say I struggled with some issues with stability with my moderately complex setup due to I believe switch placement in one area. I think it has to do with what deco wireless devices connect to then how many hops in the end they take getting to/from me to my NVR etc and back again or possible loops but basically, in short, things got better when I moved switches behind decos in each location instead of in front, then took my NVR off a switch and put directly into deco. As simply as I can put it for what pertains to this message:

Was:

Room 1

Modem -> Main Deco -> Switch 1 -> Out to Switch 2 and 3

NVR also into -> Switch 1 here

Room 2

Wall/Switch 1 -> Switch 2 -> Deco 2

Room 3

Wall/Switch 1 -> Deco 3 -> Switch 3

So as you can see Room 2 - my backhaul was going into a switch from out the wall, then the deco was plugged into the switch. I moved this from wall to Deco, then deco to switch. And, Since my new xe75 pro decos have 3 ports (I just upgraded from m5 with 2) I was able to take my NVR in main room and unplug it from switch and instead also directly into main deco. All my instability went away. (mainly, wifi cameras (POE were fine).

I know its a long shot to help you at all but my point is I had seen somewhere tp-link said to put your switches behind decos not in front whenever possible and so I made that change and boom. I also question if the Netgear switches are having issues, they are 7.5 years old, and this change maybe just went around that.

Unsure if it helps your situation, probably not, but I saw you mentioned switches and that seemed to be key to my problems. 

  @Phases Unfortunately, I can't do that.  The line between the floors is 10GBe on fiber. 

I suppose I could run a long cord up the stairs temporarily to try it out...

  @yves_b 

With the two networks configured with the same SSID, various devices connect, but the facilities that require the devices to be on the same Wi-Fi don't work.  :(

Also, I didn't see any way to ensure that my device connects to a certain band.  Can that be configured on the client side on Windows, Linux, and Android?

  @Ayengar that works too lol. I borrowed a 50ft from my work to do the same once xD

  @Ayengar Devices connected to the Deco by wire, or to a "regular" Deco wireless network (not guest, not IOT) should be working together as if they were on the same wifi.  Using the same SSID, or several SSID, should not affect that.

It is usually recommended to let the device decide which band to use.  

@ayengar While in most cases this goes without saying - make sure your firmware on all involved devices are up to date. Also in my case, I cannot have a single network with all three wifi bands, 6ghz has its own. Can you post a diagram of your network?

  @Ayengar 

You can set device band preference - to into your Deco app and where it says "xx online clients" click the link to pull connected list. You can select them then go to 'connection preference' and override auto/auto to choose which deco and which wifi band to use.